The present invention relates to a process for the autothermal catalytic reforming of hydrocarbons by passing a reactant mixture consisting of hydrocarbons, oxygen and water or water vapour, heated to a preheating temperature, over a catalyst.
For the production of hydrogen, as is well-known, hydrocarbons are reacted at high temperatures in the presence of water vapour on a suitable catalyst to give hydrogen, carbon monoxide and carbon dioxide. The reaction is highly endothermic and proceeds, for example, in accordance with the following reaction equation:

The so-called steam/carbon ratio S/C (Steam to Carbon ratio) is characteristic of this reaction. In reaction equation (1), S/C is equal to 1.
Another possibility known for the production of hydrogen is catalytic partial oxidation CPO (Catalytic Partial Oxidation). In this case, the hydrocarbons are reacted in the presence of oxygen on a catalyst, for example in accordance with reaction equation (2), to give carbon monoxide and hydrogen. An important characteristic for partial oxidation is the air index λ, which is defined as the ratio of the number of moles of oxygen used to the number of moles of oxygen required for complete oxidation (see reaction equation (3)):

The present invention deals with another possibility for obtaining hydrogen, so-called autothermal steam reforming. This process combines catalytic partial oxidation with steam reforming, wherein exothermic partial oxidation supplies the heat of reaction required for subsequent endothermic steam reforming. The reactant mixture may be preheated to a preheating temperature. The product mixture at the temperature prevailing at the reactor outlet is found in the thermodynamic equilibrium of the water gas shift reaction. Autothermal steam reforming combines the advantages of catalytic partial oxidation (good starting characteristics) with those of steam reforming (high hydrogen yields).
U.S. Pat. No. 4,415,484 discloses a catalyst for use in an autothermal reforming reactor. The catalyst contains 0.01 to 6% rhodium, as well as 10 to 35% calcium oxide on a support made from aluminium oxide which is also promoted with about 3 to 15% magnesium. The catalyst is used in the form of pellets and is characterised in particular by a low tendency to coke at low oxygen/carbon ratios. A typical catalyst system for performing autothermal reforming, according to that document, contains, on about one third of its length, an iron oxide catalyst for partial oxidation and, on two thirds of its length, the rhodium catalyst described above.
WO 98/55227 describes a bifunctional catalyst for the partial oxidation of hydrocarbons. It has a dehydrogenating activity for dehydrogenating hydrocarbons as well as the ability selectively to oxidize hydrocarbon chains. The dehydrogenation activity is provided by metals in the eighth group of the periodic system, while selective oxidation is achieved by ionized oxygen. Sources of ionized oxygen are oxides which crystallise with a fluorite structure or a perovskite structure such as, for example, zirconium oxide, cerium oxide, bismuth oxide, etc. A preferred catalyst is, for example Pt/CeGdO. It is used in pelleted form with diameters of 1.125 to 1.5 inches.
WO 99/48805 describes a process for the catalytic production of hydrogen by self-sustaining partial oxidation and steam reforming of hydrocarbons, wherein a mixture of the hydrocarbons and an oxygen-containing gas and optionally steam is reacted on a catalyst which contains rhodium dispersed on a support material which contains cerium and zirconium as cations. The catalyst is used in granulated form.
DE 197 27 841 A1 describes a process and a device for autothermal reforming of hydrocarbons in which the fuel is supplied to a two-stage reforming reactor via a feeding device. The reformate being produced is passed into a heat exchanger in counterflow and, in the heat-exchanged form, is passed into the starting substances, supplied from the outside to the inside, for reforming. The fuel supplied via the feeding device, with the starting substance, is applied directly to the reaction zone which contains a catalyst, in which combustion and reforming or catalysis is performed. The reforming reactor contains, in an upper region, a honeycomb structure coated with catalyst and, in a lower region, a packing material coated with catalyst. A honeycomb structure may also be used instead of the packing material.
Autothermal steam reforming seems to be a suitable process for producing hydrogen on board in a motor vehicle operated with fuel cells because with this process the hydrogen required for operating the fuel cells can be obtained from the fuels used for conventional internal combustion engines. An essential factor for this field of use is the hydrogen productivity which may be given with respect to both the volume of the catalyst, equation (4), and also the mass of the noble metal used, (equation (5):
                              P          Kat                =                                            V              H2                                                      V                Kat                            ·              t                                ⁡                      [                                          Nm                3                                                              1                  Kat                                ·                h                                      ]                                              (        4        )                                                      P            EM                    =                                                    V                H2                                                              M                  EM                                ·                t                                      ⁡                          [                                                Nm                  3                                                                      g                    EM                                    ·                  h                                            ]                                      ⁢                                  ⁢                              P            Kat                    ⁢                      :                    ⁢                                          ⁢                      hydrogen productivity with respect to the volume                                           V                Kat                            ⁢                                                                     of catalyst                          ⁢                                  ⁢                              P            EM                    ⁢                      :                    ⁢                                          ⁢                      hydrogen productivity with respeect to the mass of noble metal                          ⁢                                  ⁢                              V            H2                    ⁢                      :                    ⁢                                          ⁢          volume          ⁢                                          ⁢          of          ⁢                                          ⁢          hydrogen          ⁢                                          ⁢          under          ⁢                                          ⁢          standard          ⁢                                          ⁢          conditions                ⁢                                  ⁢                  t          ⁢                      :                    ⁢                                          ⁢          time                                    (        5        )            
An object of the present invention is to provide a process for autothermal steam reforming which is characterized by a very high hydrogen productivity and is thus particularly suitable for use in mobile systems.